The present invention relates to a novel heat resisting steel and, more particularly, to a heat resisting steel suitable for use as the material of blades or rotors of a steam turbine exhibiting a high creep rupture strength and toughness at temperatures ranging between 550.degree. and 600.degree. C. and having a uniform tempered martensite structure.
In recent years, there is a remarkable increase in the steam temperature and pressure at which steam turbines operate. In fact, in some steam turbines, the steam temperature and pressure well reach 566.degree. C. and 246 atg, blades and rotor shaft, therefore are required to withstand this severe condition of use. To meet this demand, hitherto, a steel called crucible 422 steel (12Cr1MoW1/4V steel) or a steel called H46 steel (12CrMoNbV steel) has been used advantageously as the material of steam turbine blades, whereas 1Cr-1Mo-1/4V steel, as well as 11Cr-1Mo-1/4V-Nb-N steel disclosed in the specification of the U.S. Pat. No. 3,139,337 has been used as the material of the rotor shaft.
On the other hand, there is a continuous and drastic rise of cost of fossil fuels such as petroleum, coal and so forth. As a result, it is becoming important more and more to increase the power generating efficiency of a power generating plant making use of such fossil fuel. For increasing the power generating efficiency, it is essential to increase the steam temperature or pressure at which the turbine operates. Unfortunately, however, known materials for steam turbines cannot be used satisfactorily under such severe conditions. Even the alloy steels mentioned above could not meet such a requirement due to insufficient high temperature strength and toughness.
Under these circumstances, there is an increasing demand for development of a material for steam turbines, having a superior high temperature strength and toughness.